Modern electronics, such as smart phones, personal digital assistants, location based services devices, digital cameras, music players, servers, and storage arrays, are packing more integrated circuits into an ever shrinking physical space with expectations for decreasing cost. One cornerstone for electronics to continue proliferation into everyday life is the non-volatile storage of information such as cellular phone numbers, digital pictures, or music files. Numerous technologies have been developed to meet these requirements.
Moreover, in the coming years, electronic systems, especially portable systems, will demand even more nonvolatile memory with high density and very high writing throughput for data storage application as well as fast random access for code execution. The flexibility and cost make the non-volatile memory a widely utilized and mature technology for most non-volatile applications.
Various types of memories have been developed in the past as electronic memory media for computers and similar systems. Such memories include electrically erasable programmable read only memory (EEPROM) and electrically programmable read only memory (EPROM). Each type of memory had advantages and disadvantages. EEPROM can be easily erased without extra exterior equipment but with reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks erasability.
A newer type of memory called “Flash” EEPROM, or Flash memory, has become extremely popular because it combines the advantages of the high density and low cost of EPROM with the electrical erasability of EEPROM. Flash memory can be rewritten and can hold its contents without power.
In Flash memory, bits of information are programmed individually as in the older types of memory, such as dynamic random access memory (DRAM) and static random access memory (SRAM) memory chips. However, in DRAMs and SRAMs where individual bits can be erased one at a time, Flash memory must currently be erased in fixed multi-bit blocks or sectors.
One popular and low-cost non-volatile memory is called “NAND” memory which is partly distinguished from other non-volatile memories because of the series connection configuration of the memory cells. Typical NAND memory is good for data storage applications but not well suited for fast random access needed for program code storage.
NAND memory uses the Fowler-Nordheim tunneling current. Programming data to the NAND memory requires high voltage, such as at least 15 volts or typically 18 volts, to store charge in the floating gate. This high voltage requirement does not scale well to smaller semiconductor geometries. The smaller and thinner physical features in the smaller semiconductor geometry process cannot reliably tolerate the high voltage levels. These additional constraints adversely impact memory density, function, performance, cost, and reliability.
Other memory approaches uses a metal-insulator-metal (MIM) structure as part of the overall memory structure. The MIM element turns on and off figuratively analogous to a mechanical switch, as the applied voltage changes and information from the MIM type memory is derived by sensing current through the MIM element. Typically, MIM type memories store data in a manner defined by the “on” or “off” state of the MIM element. Thus, the MIM element is often referred to as a MIM switch cell serving as a current switch.
In a memory array utilizing the MIM switch cell, transistors, such as a metal oxide semiconductor field effect transistor (MOSFET), serve as a transfer gate allowing access to particular portions of the memory array. Typically, the MOSFETs are connected in series to MIM switch cells as in dynamic random access memory (DRAM) type cells. However, today's technology requires approximately 100 uA to program or erase the MIM switch cell. The current required may be as low as 10 uA depending on the material used for the MIM switch cell but generally more current is necessary. Thus, the MOSFET must be designed large enough to conduct the current creating a large cell size, causing a reduction in memory density.
Resistance changing memories, including MIM memory and phase change memory, require a relatively large current to switch the memory element. This requires relatively large size switch transistors, MOSFET, in the memory cell making the cell size large. Resistance changing memories pass the read current through the switching cell. This causes so called “read disturb” to destroy the stored data as it operates.
There is an additional concern with the erasing process, since too high a voltage or too much current can damage the memory cell. In order to perform an erase without damaging the memory cell a process of erase, verify, and repeat is used. This iterative approach helps to protect the individual memory cells, but severely restricts the performance of the memory array.
Thus, a need still remains for a memory system providing low cost manufacturing, improved yields, and reduced memory cost. In view of the ever-increasing need to save costs and improve efficiencies, it is more and more critical that answers be found to these problems. Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.